The evolution and domestication of plants has generally followed a common pattern or “domestication syndrome” that distinguishes crops from their wild progenitors. One common domestication syndrome feature among crops arose from long-term selection for increased apical dominance, which is characterized by relatively more robust growth of a central stem and its buds and flowers in comparison to the growth of side stems and axillary buds, which has resulting in fewer and larger fruits per plant. The selection for apical dominance is considered an important symptom of domestication in many species, including the cereal crops of rice, wheat, barley and maize, as well as fruit crops like tomato.
A critical challenge during the domestication of crop plants was to improve the harvestability of the crop as compared to its progenitor. In unfavorable environments, wild plants often flower and mature rapidly; producing smaller numbers of branches, inflorescences, flowers and seeds in order to increase the likelihood of producing at least one offspring to continue the life cycle. In favorable environments, wild plants maximize the probability of successful reproduction by sequentially producing more branches, inflorescences, flowers and seeds over time. The latter strategy is not optimal for a crop as it is more efficient to harvest a fewer but larger fruit or inflorescences that mature synchronously from plant to plant which permits a single harvest at an optimal time of fruit or inflorescence maturation. Thus, diverse crops have been selected to produce smaller numbers of larger seeds, fruits or inflorescences on the main stem as a means of improving harvestability.
Perhaps the most striking and well-studied alteration in plant architecture was brought about by the domestication of maize. By selecting for traits that improve yield and mechanical harvestability, humans have transformed the progenitor of maize from a bushy, shrub-like ancestor with multiple elongated lateral branches tipped by male or female florescences into today's crop comprising a single, erect main stem with only two or three relatively abbreviated lateral branches, each terminating in a single female flower (ear). Today it is generally accepted that selecting for apical dominance in maize not only improves overall yield in ideal growing conditions, but it also makes the logistics of coordinating flowering times among and between lines much easier and streamlines field maintenance and mechanical harvestability.
The mechanism of apical dominance in maize involves the regulation of hormones such as auxin, which is produced by the apical meristem. As the primary ear begins to mature, greater amounts of auxin are produced by the apical meristem. The auxin is carried from the apical meristem down the plant and suppresses development of lower ears, resulting in secondary ears that are less likely to nick well or produce viable seed.
Methods for self-pollinated maize in controlled environment or nurseries have historically involved the removal of tillers and co-dominant ears because it is thought that these structures compete for resources or hormonally interfere with the primary ear and thus result in reduced grain yield. This approach likely dates back to the 1930's when corn tillers were called suckers because they were believed to ‘suck’ nutrients and sugar from the main plant. More recently research in field environments has shown that tillering is yield neutral for corn. However, farmers still prefer and select corn lines that do not tiller even in high growth environments. Both field and greenhouse plant densities are typically designed around production of plants with a single ear and tillering and ear prolificacy are often suppressed at these densities. A continuing need exists in the art for increasing seed or grain yield in monocot crop plants. In addition to broad acre yield, the ability to increase yield could be applied to breeding and increased seed production.